1. Field of the Invention
The invention is concerned with (a) rapidly solidified metal alloys useful as tool steels having composition obtained by adding small amounts of boron to alloys with compositions similar to those of commercial tool steels, especially high speed and hot work tool steels, and, (b) the preparation of these materials in the form of powder and the consolidation of these powders (or alternatively the ribbon-like material obtained from melt spinning) into bulk parts which are heat treated to uniform microstructure and desirable cutting tool properties.
2. Description of the Prior Art
Tool steels have many important metallurgical characteristics in common. In general, metal alloys useful as tool steels exhibit high hardness and resistance to abrasion as well as for many alloys the retention of these attributes at high temperatures. These characteristics are obtained by the proper choice of alloy composition, generally iron based with high carbon and alloying metal content.
One class of commercial high speed tool steels which are used primarily for cutting tools, vary in carbon content from .about.0.5 to 1.6% (wt%); in tungsten content from 0 to .about.20%; in molybdenum content from 0 to .about.10%; and in vanadium content from 0 to .about.6%. Cr is generally present at 0 to 5% and Co may be present at 0 to .about.15%. Small amounts of other elements may be present, especially Si, Mn and Ni. All high speed tool steels possess a high alloy content combined with carbon sufficient to provide excess alloy carbides in the heat treated structure and are capable of hardening to a minimum of 770 VHN (Rockwell C 63). They are hardened from temperatures within 150.degree. F. of their melting point and exhibit secondary hardening on tempering between 950.degree. to 1100.degree. F.
Obtaining the desired properties for high speed tool steels (HSTS) depends mainly upon control of the microstructure. Generally, the best properties are obtained from a homogeneous distribution of the carbides in a host structure having a small grain size. The complex chemical composition of HSTS makes the solidification process complicated and simultaneously leads to considerable phase separation during normal solidification procedures. Therefore, these steels possess a natural tendency for compositional segregation. Heterogeneity of structure and composition, particularly of carbide particle size and distribution, is one of the inherent problems in the production of HSTS by conventional practice.
In conventional practice, an as-cast ingot exhibits a microstructure of a continuous eutectic carbide network within an alloy steel matrix. The as-cast, highly segregated microstructure is then somewhat broken up by hot deformation processes. However, the final product may still exhibit heterogeneities. Also, because of hot rolling, there is a tendency for grain elongation in the rolling direction and the line up or banding of carbide particles, which leads to anisotropic mechanical properties.
In order to minimize these problems, powder metallurgical technologies have recently been applied to the production of tool steels. Powders of HSTS are produced by atomization of the molten alloy into an inert gas atmosphere or water. The faster solidification rate associated with the atomization process results in particles having a finer microstructure, i.e., a carbide morphology similar to that of the conventionally cast ingot, but with characteristic grain dimensions which are orders of magnitude smaller. The faster solidification rate also decreases the compositional segregation associated with the solidification process. The powders are subsequently consolidated into parts by conventional powder metallurgical techniques (see "High Speed Tool Steel By Particle Metallurgy" by A. Kasak, G. Steven and T. A. Neumeyer, Society of Automotive Engineers, Automotive Engineering Congress, Detroit, 1972 and "P/M Alternative To Conventional Processing Of High Speed Steels" by T. Levin and R. P. Hervey, METALS PROGRESS, Volume 115, No. 6, June 1979, Page 31.).
Because of their finer grain size more uniform dispersion of fine carbides and improved alloy homogeneity, high speed tool steels processed by such powder metallurgical techniques exhibit, compared to cast materials, superior cutting performance, a better response to hardening heat treatments, improved dimensional stability and improved grindability of cutting edges.
During the last two decades, rapid solidification processing (RSP) (also known as rapid liquid quenching (RLQ)) techniques have been used to fabricate new materials having, in some cases, new and useful properties. In RSP processes, the liquid is cooled at rates of .about.10.sup.5 -10.sup.7 .degree.C./sec and thus solidifies in a very short period of time. The rapid solidification rate leads to a microstructure and, in some cases, a metastable atomic structure, different from that obtained from standard solidification procedures. A great deal of research and development effort has been expended on amorphous metals (i.e., metallic glasses) made by a RSP process. Interesting new crystalline materials, including metastable crystalline phases, alloys having an ultrafine grain size and compositionally homogeneous alloys, can also be made utilizing a RSP process. Further, economical RSP methods for fabricating large quantities of metallic alloys in the form of filaments or strips are well established as the existing state of the art.
Metal powders when produced directly from the melt by conventional liquid atomization techniques are usually cooled three to four orders of magnitude faster than a cast ingot, although still several orders of magnitude slower than possible with RSP techniques. However, processes are now being developed for making RSP powders directly from the melt. For example, it has been reported (see D. J. Looft and E. C. Van Reuth; Proc. Conf. on Rapid Solidification Processing, p.1. Reston, VA., Nov. 1977) that rapidly solidified metal powders can be made at cooling rates in excess of 10.sup.5 K/sec by centrifugal atomization of a liquid metal stream followed by forced convective cooling. Other approaches to the production of RSP powders have been reported, for example that of Scripta Met., S. A. Miller & R. J. Murphy, Scripta Metallurgica Vol 13, PP 673-676, 1979.
Because of the potential benefits to be gained, there has been past interest in studying the effects of RSP on tool steels. I. R. Sare and R. W. K. Honeycombe applied RSP to a commercial, molybdenum rich high speed steel (AISI-M1 containing 8.4% Mo--1.5% W--4.1% Cr--1.1% V--0.77% C) using the method of "gun" splat quenching technique in which molten droplets are impact quenched against a cold metal substrate (see Rapidly Quenched Metals, N. J. Grant and B. C. Giessen, Eds., MIT Press, Cambridge, MA., 1976, pp. 179-187). The quenched high speed tool steel consisted primarily of a two phase mixture of a b.c.c. (.delta.-ferrite) phase and a f.c.c. (austenite) phase. J. Niewiarowski and H. Matyja also found a mixture of two or more phases in rapidly solidified tool steels made by a "piston and anvil" type splat quenching technique (see Rapidly Quenched Metals III, B. Cantor, Ed., The Metal Society, pp. 193-197). However, neither effort produced a homogeneous alloy. Further, neither of the processes which were used is amenable to scale up for economical commercial production.